[25] `Journal of Researches during the Voyage of the "Beagle,"
' 1845, p. 96. I have compared the rattling thus produced
with that of the Rattle-snake.

[26] See the account by Dr. Anderson, Proc. Zool. Soc. 1871, p. 196.

[27] The `American Naturalist,' Jan. 1872, p. 32. I regret that I cannot
follow Prof. Shaler in believing that the rattle has been developed,
by the aid of natural selection, for the sake of producing sounds which
deceive and attract birds, so that they may serve as prey to the snake.

It seems at first a probable conclusion that venomous snakes,
such as the foregoing, from being already so well defended
by their poison-fangs, would never be attacked by any enemy;
and consequently would have

{note [27] continued} I do not, however, wish to doubt
that the sounds may occasionally subserve this end.
But the conclusion at which I have arrived, viz. that the rattling
serves as a warning to would-be devourers, appears to me much
more probable, as it connects together various classes of facts.
If this snake had acquired its rattle and the habit of rattling,
for the sake of attracting prey, it does not seem probable that it
would have invariably used its instrument when angered or disturbed.
Prof. Shaler takes nearly the same view as I do of the manner
of development of the rattle; and I have always held this opinion
since observing the Trigonocephalus in South America.

[28] From the accounts lately collected, and given in the `Journal
of the Linnean Society,' by Airs. Barber, on the habits of the snakes
of South Africa; and from the accounts published by several writers,
for instance by Lawson, of the rattle-snake in North America,--it does
not seem improbable that the terrific appearance of snakes and the sounds
produced by them, may likewise serve in procuring prey, by paralysing,
or as it is sometimes called fascinating, the smaller animals.
no need to excite additional terror. But this is far from being the case,
for they are largely preyed on in all quarters of the world by many animals.
It is well known that pigs are employed in the United States
to clear districts infested with rattle-snakes, which they do most
effectually.[29] In England the hedgehog attacks and devours the viper.
In India, as I hear from Dr. Jerdon, several kinds of hawks, and at least
one mammal, the Herpestes, kill cobras and other venomous species;[30]
and so it is in South Africa. Therefore it is by no means improbable
that any sounds or signs by which the venomous species could instantly
make themselves recognized as dangerous, would be of more service to them
than to the innocuous species which would not be able, if attacked,
to inflict any real injury.

Having said thus much about snakes, I am tempted to add a few remarks on
the means by which the rattle of the rattle-snake was probably developed.
Various animals, including some lizards, either curl or vibrate their
tails when excited. This is the case with many kinds of snakes.[31]
In the Zoological Gardens, an innocuous species, the _Coronella Sayi_,
vibrates its tail so rapidly that it becomes almost invisible.
The Trigonocephalus, before alluded to, has the same habit;
and the extremity of its tail is a little enlarged, or ends in a bead.
In the Lachesis, which is so closely allied to the rattle-snake
that it was placed by Linnaeus in the same genus, the tail ends
in a single, large, lancet-shaped point or scale. With some snakes
the skin, as Professor Shaler remarks, "is more imperfectly detached
from the region about the tail than at other parts of the body."
Now if we suppose that the end of the tail of some ancient American
species was enlarged, and was covered by a single large scale,
this could hardly have been cast off at the successive moults.
In this case it would have been permanently retained, and at each period
of growth, as the snake grew larger, a new scale, larger than the last,
would have been formed above it, and would likewise have been retained.
The foundation for the development of a rattle would thus have
been laid; and it would have been habitually used, if the species,
like so many others, vibrated its tail whenever it was irritated.
That the rattle has since been specially developed to serve as an
efficient sound-producing instrument, there can hardly be a doubt;
for even the vertebrae included within the extremity of the tail have
been altered in shape and cohere.